I'm a newcomer to this, I lack much of the background, and I'm probably suggesting a solution that's too specific to this diamond heist scenario. But, I already spent an hour writing it down, so I might as well share it.
Trusted timestamping, cryptographically secure sensor
This is a very basic "builder move", I guess? The idea is to simply improve our sensors so that it's very hard to tamper with them, through public-private key encryption. The diamond will have a small chip that constantly sends a cryptographically-signed timestamped life signal, and the AI system will be required to keep it active. The best (and ideally, only) way to do so would be by keeping the diamond protected within the vault.
The implementation could possibly be extended to cover more weak points.
For example, you could cover the diamond with additional chips in all sides. Or you could make the chip so fragile that it breaks when the diamond is affected by strong enough vibrations (as is likely, with a diamond cutter). Or you could create more complex (but hard/impossible to tamper with) chips that continuously confirm stuff like "no object has come within 10cm of the diamond" or "the temperature remained regular" or "the weight on the pedestal is exactly X grams".
My main proposal here is the concept of having better sensors that can't have their data faked. I think with enough engineering effort you could cover enough "edge cases" that you can trust the AI system to predict robbery every time robbery happens, because a mistake/deception has improbably low odds of happening.